I didn't know that you could lat long offset the planet, but then I haven't really needed to. Useful to know. Thanks Bobby I've only done this a few times, I think Ryan Archer has written some good posts on this subject, that's where I got most of my method from.
Looking at your 4 goals Daniel:
A trick I use to mark things and also reference scale, is position a sphere usually 50m radius at the location I'm interested in. You can easily disable the node to turn it off. I've just placed one on your scene and 50m is too large, 5 or 10 should do it.
For point 3, using Global Mapper makes this so much easier. All maps have some sort of method of flattening out a spherical texture, which always brings in distortion. Google Earth has it's own unique Projection Datum to the Dem you've downloaded. Manually lining up Google Earth to Dems doesn't really work. Parts line up, then drift quite quickly. So without proper mapping tools you won't get the two to line up exactly. If this is something you and your practice want to offer your clients, I would consider in investing in GM and dual boot one of your Macs. You can get all sorts of serious data in and out of it, like shape files (vector maps created for civil interests like land usage, roads and forest areas), Lidar data and premium image content. You draw an area of interest, then export your layers of .ter and image/mask maps to use in TG at (0,0,0)
I see this is around San Diego. Below is Open Street data layered over your dem.
Exporting geometry into Modo should be quite straight forward. Position a camera to look straight down on your scene, set it to orthographic and tap in the area size you want to export. Then add a Micro exporter to your scene, give it a file name. Then go to your render node attach the new camera and check Micro Exporter in the Sequence/Output tab. Set quality low to start off with and a small image size, hit render, then see how it looks in Modo. You can always increase the poly count by increasing the image size at render. You can also use the same camera to export just a hi rez top view to texture your Modo object. It's also a good technique to get an image out to create masks in a paint package, like a driveway, lawns, etc.
Anyway that's the way I'd go. I'm sure there are other ways.
Jon